Federal law requires that all Personal Communications System networks and mobile communications networks be capable of providing location information for wireless calls made to emergency services. Under Phase II of the Federal Communications Commission wireless E911 mandate, it is required that technology be in place such that a dispatcher can know more precisely where the caller is located, a capability called Automatic Location Information.
Location technologies can be classified into at least three categories, including stand-alone, satellite-based, and terrestrial-based. A stand-alone system need not rely on multiple range measurements to determine the caller's position, but can attempt to provide the location based on power level, for example. Satellite-based systems such as Global Positioning System (GPS) provide very good accuracy and availability; however, impediments to line-of-sight can impact its usefulness. Terrestrial-based systems use existing cellular network base station transceivers (BTS's) for ground-based communications. The BTS transmits multiple ranging signals for a mobile station (or cell phone) in order to determine its location. BTS are added based upon the density of people residing in a certain geographic area. Consequently, more BTS exist in an area of higher population density than in an area of lower population density, which is optimized for communications purposes, and not for location services.
One method of determining the location of a wireless caller via a terrestrial-based system is by Time Difference of Arrival (TDOA) technology. The TDOA technique works based on trilateration by measuring the time of arrival of a mobile station radio signal at three or more separate cell sites. In a cellular system using TDOA, a caller will place an emergency 9-1-1 call that is received at several BTS, although only one BTS is assigned by the network to provide cellular communications service to the caller. A BTS is part of a BSS (Base Station Subsystem), which BSS includes the BTS and a BSC (Base Station Controller). Each BTS receiving the 9-1-1 call signal will pass it through the BSC to a MSC (Mobile Switching Center).
The MSC is part of an NSS (Network and Switching Subsystem), and is a cellular central office that can perform all switching and signaling for cellular telephones in the MSC's area by routing calls between the mobile network and the fixed telephone network (e.g., the PSTN-Public Switched Telephone Network). TDOA equipment at the MSC will determine the difference in time that the 9-1-1 signal arrived at each of three or more BTS sites and calculate the latitude and longitude of the caller based on the time difference and triangulation. The MSC then forwards the 9-1-1 call, along with the caller's location, to the emergency services dispatcher at a correct Public Safety Answering Point (PSAP).
One current method of implementing TDOA location technology in wireless networks requires that the BTS have the capability to determine timing information for received signals. In support thereof, a Wireless Location Sensor (WLS) can be located at each BTS. The WLS measures features of the wireless mobile station radio signals and transmits the 9-1-1 call signal information to a Geolocation Control System (GCS) that is also located at the MSC. The GCS is a central location processor that manages, coordinates, and administers the WLS network. The GCS converts the radio signal information received from the WLS into latitude/longitude data and communicates the data to the PSAP for deployment of emergency services.
However, providing a WLS at each BTS is very expensive due to equipment, installation, and maintenance costs. Accordingly, there is an unmet need for an improved emergency wireless caller location system.